A polymer dispersed liquid crystal display element, which is a display system utilizing the light scattering effects of the composite material comprising liquid crystal and polymer compound, requires no polarizers to produce linearly polarized light, unlike general type liquid crystal display elements such as Twisted Nematic (TN), and thus has a high light availability efficiency. Accordingly, attention is being given to the polymer dispersed liquid crystal display element as the coming generation of liquid crystal display element, and the research and development are being lively made.
The polymer dispersed liquid crystal display element can be classified under the following structure. First is the structure of Nematic liquid crystal being micro-encapsulated with polyvinyl alcohol and the like, which is called NCAP (Nematic Curviliner Aligned Phase); Second is the structure of liquid crystal droplets of a generally spherical or ellipsoid-of-revolution form being each separately dispersed in polymer matrix, which is called PDLC (Polymer Dispersed Liquid Crystal) (e.g. Society for information display international symposium digest '90 P.227-230); Third is the structure of the liquid crystal droplets existing in the form of being partly contacting with and connecting with each other, not in the form of being separate from each other (e.g. 22.sup.nd liquid crystal symposium digest 1996 P403-404); and Fourth is the structure of polymer resin spreading in the form of a three dimensional network in a continuous phase of liquid crystal, which is called PNLC (Polymer Network Liquid Crystal) (e.g. U.S. Pat. No. 5,304,323 and 15.sup.th liquid crystal symposium digest 1989 P190).
Usually, only one of these kinds of structure was adopted in these conventional type of polymer dispersed liquid crystal display elements.
Take the polymer dispersed liquid crystal display element of the conventional type having the structure of the liquid crystal droplets being dispersed in the form of a part thereof being contacting with and connecting with each other (see FIG. 31(a)) for instance, it was produced by the following technique.
First, opposing upper and lower substrates 1001 and 1002 were bonded together through a sealant 1006 so that an uniform gap could be formed therebetween. Then, a mixture including liquid crystal material and polymerizable monomer was filled in between the upper and lower substrates 1001 and 1002. After polymerization temperature and irradiation intensity of ultraviolet was so set as to be prescribed conditions,. the mixture was irradiated with ultraviolet, so that the monomer is polymerized to cause a phase separation of the liquid crystal material. The irradiation of ultraviolet was so controlled as to be uniform in the panel surface.
As a result of this, the state of the liquid crystal material being dispersed in the polymer matrix or the state of the liquid crystal material being dispersed in continuation in the polymer matrix was presented between the two substrates in accordance with the prescribed conditions (cf. e.g. Flat panel display '91, NIKKEI BP Co., Ltd., Page 221).
However, the polymer dispersed liquid crystal display element having the structure of the liquid crystal droplets being completely separated from each other, which is in actual use in the TFT liquid crystal panel, is low in light scattering, thus presenting problems of low contrast and high driving voltage.
The reduction of scattering of light is caused for the following reason. In the case of liquid crystal droplets being separated from each other, their particle size is about 0.8 .mu.m, which corresponding to about 69% of the percentage of the liquid crystal. If the particle size and percentage of the liquid crystal exceed these values, the liquid crystal droplets will take the form of being partly connected to each other. Now, if incident light has wavelength of, for example, about 0.4 .mu.m, the particle size of the liquid crystal droplets of about 1.2 .mu.m, which corresponding to about 75% in the percentage of the liquid crystal, is required for the sufficient effect of scattering of light. However, in the case of the liquid crystal droplets being separated from each other, as described above, since the particle size is too small, the scattering of light is low and thus deterioration of contrast is caused. On the other hand, the increase of the driving voltage is caused for the following reason. In the case of the liquid crystal droplets being separated from each other, since the scattering of light is low, as mentioned above, the panel gap must be enlarged to obtain the same scattering of light as in the case of the liquid crystal droplets being partly contacting with and connected with each other. As a result of this, the increase of the driving voltage is caused.
The polymer dispersed liquid crystal display element having the structure of the liquid crystal droplets being partly connecting with each other or the polymer network type of polymer dispersed liquid crystal display element has a problem that cracks develop in the polymer resin extending from an area in the vicinity of the sealant to an active area due to variations in ambient temperature and thereby stripy display unevenness is caused.
The inventors discovered that the display unevenness was caused in, for example, the inspection step in which the polymer dispersed liquid crystal display element is allowed to stand under environment of high temperature for a given time and thereafter is cooled down to room temperature, for the purpose of evaluating the reliability in temperature variations. The mechanism for causing the display unevenness will be described below. When the polymer dispersed liquid crystal display element is allowed to stand under high temperature of 80.degree. C. for 24 hours, polymer resin 1005 and liquid crystal 1004 expand, as shown in FIG. 31(b). At that time, the sealant 1006 supporting the upper and lower substrates also expands with temperature, but is rather smaller in degree of expansion. Due to this, the upper and lower substrates of the liquid crystal panel deforms into a convex form in section. At high temperatures at which viscosity of liquid crystal is reduced sharply, the liquid crystal flows with ease. In addition, since the upper substrate 1001 and the lower substrate 1002 are fixed via the sealant 1006 around their marginal portions, a pressure from the upper substrate 1001 and the lower substrate 1002 is applied to a composite layer in the direction indicated by arrows in the same figure. As a result, the liquid crystal existing in the vicinity of the sealant 1006 receives the pressure and flows to an interior of the panel. Then, when the liquid crystal panel is cooled down to room temperature, the viscosity of the liquid crystal increases under room temperature. As a result of this, the liquid crystal flowing to the center part of the liquid crystal panel does not return to the area in the vicinity of the sealant 1006, and resultantly the density of liquid crystal in the area in the vicinity of the sealant 1006 decreases. Because of this, when a pressure from the upper substrate 1001 and the lower substrate is applied to the polymer resin matrix in the vicinity of the sealant 1006, cracks 1007 develop. See FIG. 31(c). Thus, the stripy display unevenness is created around the marginal portion of the display screen.
As discussed above, with the polymer dispersed liquid crystal display element having the structure of the liquid crystal droplets being completely separated from each other, the stripy display unevenness is not visually confirmed on the display screen, but contrast is originally low. On the other hand, with the polymer dispersed liquid crystal display element having the structure of the liquid crystal droplets being partly connected with each other or the polymer network type of polymer dispersed liquid crystal display element, good contrast is presented, but the stripy display unevenness is visually confirmed on the display screen. Thus, the polymer dispersed liquid crystal display element that can provide good contrast and also can prevent the development of display unevenness has never been provided so far.
On the other hand, the stripy display unevenness caused by the development of cracks can be created due to the following mechanism. It is noted that the display unevenness stated herein can be created in the structure of liquid crystal droplets of being each separately dispersed in polymer resin matrix, which is called PDLC, as well. The discussion on the mechanism will be given below, taking the PDLC as an example.
The above-described polymer dispersed liquid crystal display element has the structure in which a composite layer 2103 is interposed between upper and lower substrates 2101, 2102 forming thereon display electrodes 2104, 2104, as shown in FIG. 32. A color filter layer 2106 is formed between the upper substrate 2101 and the display electrode 2104. The composite layer 2103 is formed with being in intimate contact with the upper substrate 2101 and the lower substrate 2102. The composite layer 2103 has the structure of the liquid crystal droplets being dispersed in the matrix phase of polymer compounds.
The above-described polymer dispersed liquid crystal display element has a problem that the composite layer 2103 expands and contracts due to variations of ambient temperature, to cause the cracks in the polymer resin in an area in the vicinity of the sealant 2105 and in turn cause the stripy display unevenness.
This stripy display unevenness is caused in, for example, the inspection step including a heat shock process in which the polymer dispersed liquid crystal display element is allowed to stand under environments of high temperature for a given time and thereafter is cooled down to room temperature, for the purpose of evaluating the reliability in temperature variations. Specifically, when the polymer dispersed liquid crystal display element is allowed to stand under high temperature for a given time, the composite layer 2103 expand, as shown in FIG. 33(b). At that time, the viscosity of the composite layer 2103 decreases sharply under this environment, so that the flowability of the composite layer 2103 increases. In addition, since the upper substrate 2101 and the lower substrate 2102 are fixed via the sealant 105 around their marginal portions, a pressure from the upper substrate 2101 and the lower substrate 2102 is applied to a composite layer 2103 in the direction indicated by arrows in the same figure. As a result of this, polymer resin and the others flow to a center part of the liquid crystal panel, to cause the center part of the panel to expand further (see FIG. 33(c)). Then, when the liquid crystal panel is cooled down to room temperature, the viscosity of the composite layer 2103 increases and thereby the flowability decreases. Due to this, the density of liquid crystal existing in the vicinity of the sealant 2105 is reduced, to thereby cause the cracks 2110 to develop in the vicinity of the sealant 2105 (See FIG. 33(d)). As a result of this, the stripy display unevenness is created around the marginal portion of the display screen.
Further, in the case where the composite layer 2103 is in intimate contact with the upper and lower substrates 2101, 2102, as in the conventional type of polymer dispersed liquid crystal display element described above, there arises a problem that when the display screen is pressed through, for example, a pen input and the like, the display unevenness is created on the display screen.
This problem will be discussed in detail below. FIG. 34 is a plan view of a conventional type polymer dispersed liquid crystal display element when viewed from the upper substrate 2101 side. When the liquid crystal panel is pressed at the point A shown in the same figure from the lower substrate 2102 side, the composite layer 2103 which is in intimate contact with the upper and lower substrates 2101, 2102 deflects together with the upper and lower substrates 2101, 2102 (See FIG. 35). This produces shearing stress 2122 in the directions indicated by arrows in the figure in the most deflected part of the liquid crystal panel between the upper substrate 2101 and the composite layer 2103 and between the lower substrate 2102 and the composite layer 2103. Through the action of shearing stress 2122, microscopic spaces surrounded by the polymer resin are each deflected from their original spherical form to a flattened form having a length extending vertically to the upper substrate 2101 which is smaller than a length extending parallel to the same 2101 (See FIG. 36). As a result of this, the liquid crystal molecules enclosed in the microscopic spaces of the flattened form are aligned in the shearing stress direction. Thus, when the electric field is OFF, in an area 2111, transmitted light is scattered and opaque appearance is presented, while on the other hand, in an area 2110 of a wing-like form, the index of refraction with respect to the direction of incident light decreases and thus the scattering of light is weakened, so that somewhat transparent appearance is presented. As a result, the display unevenness is created on the display screen (See FIG. 34). On the other hand, when the electric field is ON, a threshold voltage in the area 2110 of the wing-like form becomes smaller than that in the area 2111, so that variations in threshold voltage are caused. Thus, in this case also, the display unevenness is created.
Further, the above-described conventional type of polymer dispersed liquid crystal display element has the problem of generation of color mixing among coloring material layers R, G, B and loss of light availability efficiency caused by black matrixes.
Specifically, as shown in FIG. 37, the light coming from, for example, the lower substrate 2101 side is scattered when it is incident on the composite layer 2103. A part of the incident light as scattered is then absorbed in the black matrixes to cause the loss of light. On the other hand, the remaining light passes through the composite layer 2103 and arrives at the coloring material layer G of the color filter layer 2106. Further, when the light is incident on the upper substrate 2101 from the color material layer G, the light is scattered radially. Where the upper substrate 2101 is made of glass or equivalent, the index of refraction therein n.sub.g (e.g. n.sub.g =1.5) is larger than the index of refraction in air n.sub.air (=1.0). Due to this, some of the scattered light is totally reflecting at the boundary between the upper substrate 2101 and the air. When the totally reflecting light is incident on the adjacent coloring material layer B, the color mixture is caused.
Furthermore, the above-described conventional type of polymer dispersed liquid crystal display element has the following problem as well. With the electric field applied to the composite layer 2103, the state of display is inspected on whether there exist defects such as dot defects and line defects. This inspection step is not performed until after an assembled empty cell is filled with a liquid crystal material to form the liquid crystal cell (the same is applied to the other conventional types of polymer dispersed liquid crystal display elements including TN type one). Therefore, when defects of the display screen originating from defects of the composite layer 2103 are found in the inspection step, even the opposite substrate with an expensive color filter layer is junked, thus introducing the increase of costs.
To sum up the foregoing, the above-described conventional type of polymer dispersed liquid crystal display elements have the following disadvantages.
In the case of the polymer dispersed liquid crystal display element having the structure of the liquid crystal droplets being completely separated from each other, light scattering is low and thus contrast is poor and also driving voltage is high; PA1 Display unevenness is caused by deflection of the liquid crystal panel or by heat shock or equivalent in the reliability evaluation test; PA1 When the color filter layer is provided for performing color display, the color mixture is caused among the coloring material layers of R, G, B and reduction in light availability efficiency caused by the loss of light through the black matrixes is introduced; and PA1 When defects are found by the inspection of display, even the opposite substrate with the expensive color filter layer is junked, thus introducing the increase of costs.